An X-ray detector, in particular a quanta-counting X-ray detector, is used in imaging applications. An X-ray detector of this kind is used by way of example therefore for computed tomography recordings in medical imaging to generate a spatial image of an examination region of a patient.
An X-ray detector, whose sensor layer is designed as a direct-converting semi-conductor layer, enables a quantitative and energy-selective detection of individual X-ray quanta. Pairs of electron holes, i.e. pairs of negative and positive charge carriers, are generated in the sensor layer on the penetration of X-ray radiation. The charge carriers are separated and move to the electrodes or surfaces of the sensor layer with the opposite charge respectively due to a voltage applied to the sensor layer or surface of the sensor layer. The current caused as a result, or a corresponding charge transfer, can be evaluated by an electronic sensor device connected downstream. Semi-conductor materials by way of example in the form of CdTe, CdZnTe, CdTeSe, CdZnTeSe, CdMnTe, GaAs, Si or Ge, which have a high absorption cross-section for X-ray radiation, are suitable for detection of the X-ray quanta.
Large-area X-ray detectors are required in particular for a computed tomograph, for which reason a plurality of comparatively small detector modules with the above-described construction is frequently arranged side by side. These detector modules typically have a sensor surface between 1 cm2 and 4 cm2. To achieve optimally high image quality the detector modules are also arranged with the smallest possible spacing from each other. The voltage applied to the sensor layer or to the sensor surface is fed to the detector modules of an X-ray detector by way of individual power supply channels of the HV supply and is adjusted to a predefined operating voltage.
In the case of direct-converting X-ray detectors, or detector modules with a corresponding construction, the electrical resistance of the sensor material changes with the X-ray flux. This leads to a change in the power loss. A change in the X-ray flux therefore causes a change in temperature in the sensor layer, whereby the energy resolution and the count rate of the X-ray detector are in turn affected. A temperature-dependent count rate drift is a miscount that cannot be corrected and leads to image errors and artifacts in the tomographic scans created from absorption data.
In addition to a change in the temperature of the sensor layer caused over time, the drift behavior of an X-ray detector is also affected by locally different temperatures of the sensor layer. Temperature gradients of this kind result in particular due to uneven heat dissipation in the sensor layer.
An undesirable temperature gradient can also result on the sensor board or in the sensor layer of the corresponding sensor board depending on the operating point of the sensor board. An elevated current through the sensor material which, even without penetrating X-ray radiation, can lead to a high power loss and therefore also to a temperature gradient in the sensor layer, can occur as a function of the respectively chosen setting of the operating parameters, such as the mean operating temperature of the detector module or of the sensor material or the applied supply voltage.
To avoid temperature gradients in the sensor layer, preferably all-over thermal coupling to a heat sink is desirable. In current detector modules cooling is implemented by way of example by cooling ribs in a module support to which the sensor layer can be coupled in a stack formation by way of a support ceramic. The sensor layer can therefore be uniformly heated with all-over coupling of the support ceramic to the module support.
However, all-over coupling is made difficult by components arranged on the bottom of the support ceramic, for example components such as passive elements or connectors for data transfer, and/or by other mechanical indentations used for connection to an electronic sensor device. Since these components impede uniform heat dissipation in the sensor layer the regions of the sensor layer, which are applied to the regions of the support ceramic provided with the components, have a slightly higher temperature during operation than the area that is free from the elements at which the support ceramic is coupled to the module support in a planar manner.
One possibility for avoiding this problem can be achieved by way of example by a change in the geometry of the support ceramic. A support ceramic can therefore be used by way of example which is only partially covered by the sensor layer. On account of its larger area the support ceramic then provides “free” regions to which the above-mentioned components can be attached.
An embodiment of this kind is not possible, however, by way of example in a detector module with a tiled modular construction, without restrictions. In the case of a tiled detector module a plurality of tiles, what are known as sensor boards, are arranged adjacent to each other on a shared module support at a spacing of about 100 μm, with the sensor layers of the sensor boards jointly forming the sensor surface of the detector module. A limitation of the spatial resolution must be accepted with a change in the geometry as described above. The number of sensor boards arranged side by side on a module support would also be reduced owing to the larger area.